#include<iostream>
#include<vector>

int takeAttendance(std::vector<int>& records) {
    int left = 0, right = records.size() - 1;
    if (records[right] == right)
        return right + 1;
    while (left < right)
    {
        int mid = left + (right - left) / 2;
        if (records[mid] == mid)
            left = mid + 1;
        else
            right = mid;
    }
    return left;
}